GPU + CPU rendering (2.80)
You could choose to use 1 thread, but that is a really bad idea. It will render slower and will not use the CPU to its full potential.

Try to optimize the tile size instead. Try small sizes like 16x16 as well. Don't worry about the GPU - GPU rendering is a lot more optimized for small tile sizes in 2.80. 



See what uses the CPU most efficiently and renders fastest. You cannot use multiple CPU cores on one tile efficiently.